1. Field of the Invention
The present invention relates to a scroll-type compressor with a variable displacement mechanism. More particularly, it relates to a scroll-type compressor with a variable displacement mechanism for which the minimum operating capacity is improved.
2. Description of the Related Art
Generally, a method of returning a portion of refrigerant gas in the compression chamber to the suction chamber is known in the field of scroll-type compressors. FIG. 1 is a cross-sectional view of a prior art conventional compressor 1' according to Japanese Patent Publication Hei 5-280476. In FIG. 1, the capacity control mechanism 600 is comprised of cylinder 510, which is formed within the end plate 501 of fixed scroll 500; a plurality of bypass holes 530, which allow compression chambers 520a, 520b to be in communication with the cylinder 510; a plunger 540, which can open or close bypass holes 530 sequentially; and a mechanism, which regulates the position of plunger 540 along the axis of the cylinder 510. The outermost one of bypass holes 530 permits cylinder 510 to be in communication with suction chamber 550. The mechanism that regulates the position of plunger 540 is comprised of control valve assembly 560, control pressure chamber 570, spring 580, and stopper 590. Control valve assembly 560 regulates the pressure in control pressure chamber 570, so as to increase that pressure when the thermal load for the air conditioning system is high, and decrease it when the thermal load is low. Accordingly, when the thermal load is high, plunger 540 is pushed in a radially outward direction within the compressor by the pressure in control pressure chamber 570, so that bypass holes 530 are closed sequentially. As a result, the return of the refrigerant gas from compression chambers 520a, 520b to suction chamber 550 is blocked, and the compressor operates at its maximum capacity. When the thermal load is low, the force exerted by spring 580 overcomes the counter force exerted by the pressure in control pressure chamber 570, and, therefore, plunger 540 is pushed in a radially inward direction within the compressor, so that bypass holes 530 open sequentially. As a result, the return of the refrigerant gas from compression chambers 520a, 520b to suction chamber 550 is allowed, and the capacity of the compressor is decreased automatically.
When the thermal load is very small, plunger 540 is in the most recessed position within cylinder 510, opening all bypass holes 530. In this state, part of the refrigerant gas in compression chamber 520a, for example, returns to suction chamber 550 via the path L1' as indicated in FIG. 1. The compressor is expected to operate at about its minimum capacity, for example, at about 25 percent of the full capacity of the compressor.
However, in a compressor according to the prior art, minimum operating capacity does not decrease to the 25 percent due to the prior art's design. The design impedes the compressor from going down to its expected lower limit of capacity, due to path resistance against the returning gas from compression chambers 520a, 520b to suction chamber 550. The path resistance is affected by various factors, such as the diameter of bypass holes 530, the cross-sectional area of cylinder 510, the length of the path, and the bendings of the path for the returning gas. This phenomenon of path resistance manifests itself as a large pressure loss which means that the pressure difference between the compression chamber, from which the returning gas departs, and the suction chamber, which receives the returning gas, is large. For a long time, it has been desired to reduce the pressure loss of returning gas in a capacity control mechanism in order to secure a sufficient quantity of returning gas and to realize the expected minimum capacity.
There are physical restrictions, however, that limit the ability to improve path resistance. For example, the diameter of the bypass holes may not be larger than the thickness of the spiral element 502 without causing undesired communication between neighboring compression chambers when the bypass holes are closed by plunger 540. Similarly, the cross-sectional diameter of cylinder 510 may not be any larger than the thickness of end plate 501 of fixed scroll 500. Moreover, if the thickness of the end plate 501 is increased for the purpose of providing a larger cross-sectional diameter of cylinder 510, the size in the axial direction of the compressor and weight of the compressor are undesirably increased.